Blow molded thermoplastic containers comprised of polyethylene terephthalate (PET) are predominantly made in one of two ways: (a) two-stage injection-stretch blow molding or (b) single-stage injection-stretch blow molding. Injection-stretch blow molding processes generally fit into one of two categories.
In a first type of process, a preform is injection molded, allowed to cool, stored, and is later blown to the shape of the desired article. Prior to blow molding, however, the preform is heated to its blow molding temperature. This type of process can be, inter alia, energy intensive.
In the second type of injection-stretch blow molding process, the preform is injection-molded and transferred, in a heated condition, to a blow station where it is blown. This second type of process requires less energy than the first type of process; however, both processes may be inherently limited in certain respects because the injection molding step is intermittent, i.e., the molten thermoplastic is injected as a shot into the preform mold, which is not a continuous operation. Another consideration associated with injection blow molding is the high cost of tooling and capital for the die head and preform mold. New or reworked die and preform molds are often required for each different shaped article produced because the preform from which the desired article is blown generally must meet very specific dimensional requirements to form the desired final blow molded article.
Another method for producing thermoplastic articles is extrusion blow molding. Examples of this method are disclosed in the art. In this type of process, the thermoplastic material is continuously extruded in the form of a hollow tube, segments of the tube are enclosed in a blow mold, and the desired article is blown. Compared to PET, many commonly used polymer materials, such as PETG (a copolymer of ethylene terephthalate and diethylene glycol), have comparatively limited use. Among other things, such polymer materials may have insufficient moisture impermeability, may not pass migration tests, or may not provide a desired surface finish or clarity—particularly when compared to the finish and clarity of PET articles that have been injection-stretch blow molded.
PET resins offer a number of desirable characteristics and, as taught in certain prior references, PET containers can be formed using extrusion blow molding processes. PET is an excellent molding compound for making hollow articles, such as beverage containers; possesses good impact strength; and can be formed into clear or colored articles, as desired. In addition, PET offers good fatigue resistance and therefore its uses include applications where the product is subjected to fatigue inducing conditions—such as squeeze bottles. It is known that PET containers can be produced using an extrusion blow molding process. However, because that process does not impart the same biaxial orientation as an injection-stretch blow molding process, there may be perceptible differences, such as between the surface finishes and article clarity. Therefore, it is desirable to be able to produce an extrusion blow molded container, including PET containers, which, among other things, may exhibit improved structural strength, structural integrity, surface finish and/or clarity.